1. Field of the Invention
The present invention relates to method and apparatus for controlling temperature in a rapid furnace.
2. Description of the Related Art
Various kinds of heat treatment apparatus are used to form a diffusion layer, silicon oxide and nitride films on each semiconductor wafer and glass substrate in them. The CVD, oxide film forming and heat diffusion treatment apparatuses are used in the course of making semiconductor devices, for example.
Jpn. Pat. Appln. KOKAI Publication No. Hei 3-82016, for example, discloses a heat treatment apparatus of the vertical type in which a plurality of wafers are processed in a batch. When this apparatus is to be used to apply impurities diffusing process to the wafers, supply and exhaust openings of the vertical heat treatment furnace are closed by shutters and the wafers are loaded together with the boat into the process tube. When the inside of the process tube is then heated to a process temperature by heaters and process gas as introduced into the process tube, process gas acts on the wafers to diffuse impurities into them.
As integrated circuits are made higher in their process speed and their integration is also made higher, it is asked that the depth of diffusing impurities into the surface of each wafer is made as small as possible. In order to control this impurities diffusing depth, it is needed that the wafers are heated to a temperature (or 1000.degree. C., for example) higher than the intended one and that their predetermined temperature gradient, that is, the time during which they are kept at the intended temperature (or 500.degree. C., for example) and their temperature falling time are controlled by forced cooling. Conventionally, fans were arranged in supply and exhaust openings of the vertical heat treatment furnace for this purpose to forcedly introduce cooling air into the space between the process tube and the outer housing through the supply openings.
When the wafers are forcedly cooled by the fans in this manner, however, the temperature of the wafers becomes remarkably different from that of the heater (more specifically, the temperature of the thermocouple arranged in the heater section) and it thus takes a long time (20 minutes) that the wafers and the heater are cooled to same temperature (25.degree. C.). As shown in FIG. 1, therefore, the heater is forcedly air-cooled to lower heater temperature T.sub.H to 500.degree. C. by the fan for an initial cooling period C.sub.1 starting after the heater power source is turned off, and the ON-OFF operation of the fan is then repeated to control the heater temperature T.sub.H at about 500.degree. C. for an another cooling period C.sub.2 following the initial one C.sub.1. This ON-OFF operation of the fan is repeated until wafer temperature T.sub.W falls to 500.degree. C. Conventionally, the ON-OFF operation of the fan must be frequently carried out as described. In addition, temperature dropping control takes a long time. The productivity of wafers is thus lowered.
Further, a plurality of wafers are processed at the same time in the vertical heat treatment furnace of this kind. Therefore, wafers located at upper (top), center or lower (bottom) portion (zone) are made different in temperature from those located at other zones. The productivity of wafers is thus lowered.
In a heat treatment apparatus disclosed by Jpn. Pat. Appln. KOKAI Publication No. Hei 4-186616, a uniformly-heating member made of SiC is interposed between a heating resistance and the process tube to prevent the wafers from being heated to temperatures quite different at the top, center and bottom zones in the process tube. This uniformly-heating member serves to uniformly apply heat to all of the wafers in the process tube and also to prevent unnecessary matters spread from the heating resistance and a heat insulator from entering into the process tube.
A heater element made of FeCrAl alloy is usually used as the heating resistance in the heat treatment apparatus. The FeCrAl alloy heater element has a surface load of about 2 W/cm.sup.2 at 1200.degree. C. Recently, however, a heat treatment apparatus capable of attaining a temperature raising and dropping speed of at least 30.degree. C./minute (more preferably 100.degree. C./minute (at temperature raising time) and 60.degree. C./minute (at temperature dropping time)) is expected and attention is now paid to molybdenum disilicide which has a surface load (of 20 W/cm.sup.2) larger than that of the FeCrAl alloy heater element.
When the uniformly-heating member is interposed between the wafers and the heating resistance, however, the temperature raising speed of the wafers is made lower for the initial period. In addition, the uniformly-heating member makes it more difficult for infrared rays emitted from the heating resistance to enter into the process tube through it.
When no uniformly-heating member is used, metal ions such as Fe, Cu and Na caused from the heating resistance and its support enter into the process tube through the quartz-made wall thereof to thereby contaminate the wafers in the tube by heavy metal.